This invention relates to television time-base correctors and synchronizers in which an analog television video signal is digitized at a rate determined by the input signal subcarrier to form digital words which are written into a memory and which are read out of the memory in synchronism with a local reference subcarrier.
In the recording of video signals by video tape recorders, video discs and the like, the video information is encoded and recorded on a moving mechanical medium such as tape or discs. Unavoidable fluctuations in the mechanical speed of the medium during both recording and playback cause changes in the data rate of the video on playback compared with that rate at which it was recorded. The use of servo-mechanisms to control the speed of the medium can maintain a correct average data rate on playback, but short-term fluctuations in the mechanical speed of the medium cause short-term fluctuations or jitter in the video signal on playback. Such jitter may be tolerable for some applications, but electronic time-base correctors are used to correct the jitter for critical applications.
Such time-base correctors include a digital memory capable of storing the information contained in several horizontal lines (one or more vertical fields in the case of synchronizers) of video information. An analog-to-digital converter (ADC) converts the incoming video to a stream of digital words which are written into the memory for storage. At a later time, the digital words are read out of the memory at a constant clock rate established by a local reference source, thereby eliminating the jitter.
The chrominance information contained in the television video signal is encoded as phase and amplitude modulation of a signal relative to a color reference subcarrier. Since there is no reference subcarrier which is common to both the input and output sides of the memory, the chrominance information is maintained by making the write-in and read-out clock rates the same multiple (such as 3 or 4 times) of the input video and local output reference subcarrier frequencies, respectively.
A saving of memory capacity may be achieved by storing only the active video information and not storing the repetitive synchronizing information, as described in U.S. Pat. No. 4,101,926 issued July 18, 1978 to Dischert, et al. In this arrangement, maintenance of the color information between input and output of the memory is accomplished by beginning the clocking for write-in and read-out of each horizontal line in a coherent manner, at a fixed preassigned phase of the relevant subcarrier signal.
In such coherent memory arrangements, a subcarrier signal is phase-locked line by line to the color burst signal of the incoming video. This signal is a reconstruction of the original subcarrier that existed when the video and burst were generated and is referred to hereinafter as a WRITE subcarrier. Digitizing and writing into the memory for each successive horizontal line begins at the first occurrence of a particular phase of the WRITE subcarrier occurring after a fixed interval after the horizontal synchronizing signal. For example, clocking for each horizontal line might begin at the first positive-going zero crossing of the reference subcarrier occurring after a time 8 microseconds after each horizontal synchronizing pulse of the incoming signal. Similarly, clocking for read-out from the memory in this case would begin at the first positive-going zero crossing of the local reference subcarrier which occurs after a time 8 microseconds after the local reference horizontal synchronizing signal. This color phase information is not lost in translation through the memory.
Television standards do not closely specify the timing of the color burst with regard to the horizontal synchronizing signal. Consequently, video from two different sources may have differences in the phase of the color burst signal relative to the horizontal synchronizing signal even though the burst frequencies of the two sources are identical and the synchronizing signals are coincident. In a coherent system such as that of Dischert, et al., a change from one source of video to another can result in a change in the phase that the input WRITE subcarrier takes at a fixed time after the horizontal synchronizing signal. The next following occurrence of a positive-going zero crossing of the WRITE subcarrier may thus cause a change in the time of commencement of gating for writing into the memory compared with the time of commencement of the previous line. The maximum time change is equal to the duration of one cycle of color subcarrier or 280 nanoseconds. The read-out, however, continues unchanged. The change in delay between the horizontal synchronizing pulse and the beginning of writing-in to the memory thus results in a horizontal displacement of the video as displayed on a raster. A change of 280 nanoseconds is highly visible. While a single such change during switching between two sources might not be objectionable, in stop-motion applications in which less than 4 NTSC fields are stored and successively displayed, horizontal displacement of the video of one field will occur relative to the video of the preceeding and succeeding fields. Thus, continuous horizontal displacement at the field rate occurs, which is highly objectionable. If an attempt is made to reduce the step size, chroma errors occur between successive fields in such stop-motion systems.